The present invention relates to photodetector apparatus for generating a signal as a function of light intensity and, more particularly, to circuits generating a digital value reflecting light intensity.
A major area of need for the benefits of the present invention is that of spectroradiometry. As long as there have been artificial light sources, there has been a need to measure and quantify intensity and color, the two basic attributes of light. Color is a spectral quality of light, and the makeup of its wavelengths and corresponding amplitudes comprise the spectral distribution, a typical plot of which is shown in FIG. 1. This spectral distribution is responsible for determining the singular or varied hues and intensities that make up a visual color response.
Strictly speaking, spectroradiometry is the measurement of the spectral distribution of light sources. Interpreting the spectral distribution plot in FIG. 1 points up the psychophysical (visual) importance of spectroradiometry. Here, four narrow lines of very precise wavelength have amplitudes well above the mean signal level, which indicates that there are some very distinct spectral components in the visible range--four distinct colors. It also demonstrates the extremes of intensity that are encountered in such measurements--from virtually 0% to 100% in one sample.
The human eye cannot resolve these spectral lines, requiring instead an instrument to split light into precise spectral components. A conventional photometer with a series of narrowband filters that pass, say, only 5 nm difference in wavelength could do this, but obviously if we don't know the location of the spectral lines, a search of the complete spectrum from 350 to 730 nm with a 5 nm bandwidth would require 80 separate filters--an extremely expensive and time-consuming task. Of the many methods now in use for measuring light intensity and color, spectroradiometry appears to be the best.
In the single detector type spectroradiometer, mechanical deflection is used to sweep the spectrum of the incoming light beam across a single photodetector. Thus, the spectrum intensity data is generated serially at the output of the photodetector. Although very accurate and senstive, mechanically scanned, single-detector spectroradiometers have one major disadvantage--slow speed. Mechanical scanning of a complete spectrum can take anywhere from 15 seconds to 15 minutes, depending on spectral resolution, sensitivity and other instrument parameters. During this time, problems can occur as a result of light source instability or motion. Of course, the mecchanically scanned type also can't be used for measuring a single short flash of light.
A parallel-acquisition device--one that acquires a complete spectrum at the same instant (i.e., a spectrometer)--will eliminate these problems. It may be a Fourier spectrometer, which is of practical interest only in the infrared range, or a spectrograph, which uses a photographic material as the radiation detector. The most useful, however, is the multiple-detector spectroradiometer, which was introduced commercially around 1970.
Construction of a multiple-detector spectroradiometer is much like that of a conventional one in that it uses a wavelength disperser, such as a diffraction grating. However, unlike the monochromator in the conventional type, the multiple-type detector design does not use an exit slit to isolate a single wavelength of light. Instead, the radiation detector itself is placed at the exit focal plane of the disperser so that each element of the multiple-element detector captures a narrow band of wavelengths, acquiring the entire spectrum at once. The primary advantage is that the entire spectrum is acquired in parallel at the same instant, which enables these instruments to measure brief flashes of light or light sources that vary with time.
A multiple-detector spectroradiometer, generally indicated as 100, is shown in simplied form in FIG. 2. The wavelength disperser, generally indicated as 102, in the multiple-detector spectroradiometer 100 is usually referred to as a spectrograph or a polychromator. The optical elements are similar to those in a monochromator, except that they must be optically corrected for a much larger image size. In the wavelength disperser 102, the incoming radiation 104 passes through the entrance slit 106, is collimated by lens 108 and falls on the diffraction grating 110. The diffracted spectrum 112 is focused on the multiple-element photodetector 114 so that each end of the spectrum is aligned with each end of the photodetector 114. The output appears on line 116. Individual elements of the detector 114, as many as 2000 or more, resolve all intermediate spectral bands. As a practical example, a 256-element detector can cover the visible spectrum from 380 to 740 nm with a spectral resolution of 1.4 nm per element. The multiple-element photodetector 114 can be any radiation detector that is either segmented or has spatial resolution capability.
As will be easily recognized, if the multi-element photodetector 114 is connected to a prior art control system such as that of FIG. 6, to be described hereinafter, the possibility and probability of losing valuable data is very high.
Digital image processing is another rapidly expanding art requiring the benefits of the present invention. By such techniques, a photograph is scanned and the information thereon put into digital form. The digital data is then manipulated in a digital computer to enhance the data according to preselected criteria. The enhanced digital data is then used to regenerate an enhanced photograph. For example, an out-of-focus photograph or one streaked by movement of the camera can be digitally processed to improve its resolution and to restore lost details. In many cases, the cause of the blur can be determined from the blurred image itself.
As with most data based systems, the total system is limited by the original input of data. A digital computer processes information in discrete numerical units. Most images, of course, do not come in such units. An ordinary photograph is an analog representation of a scene. The information is recorded in a continuous gradation of tone across the two-dimensional surface of the film. Processing a photograph by means of a computer therefore requires that the analog image first be converted into a digital one.
There are a number of ways of digitizing an image. One example is shown in FIGS. 3 and 4. The system, generally indicated as 10, is what is referred to as a microdensitometer. In such a system, light source 12 projects a beam of light 14 which passes through lens 16 to create a spot of light which passes through the transparency 18 and thereafter strikes a photomultiplier 20 creating an analog signal such as that indicated as 20 at the output thereof. The output of the photomultiplier 20 is connected as an input to a quantizer (A/D converter) 24 which creates a digital signal such as that indicated as 26 at its output. The output of the quantizer 24 is connected to a digital computer 28 which enhances the data according to preset algorithms and stores it for later use. The computer 28 can then take the enhanced data and produce an enhanced digital signal, indicated as 30, which can be used to drive a modulated light source 32 to create a modulated beam of light 34 which passes through focusing lens 36 to create a modulated spot of light on the surface of a film sheet 38 which can then be developed to create an enhanced version of the original photograph on the transparency 18. Both the transparency 18 and the film sheet 38 are moved in a raster scan pattern as indicated by the arrows 40 and 42, respectively.
Turning to FIG. 4, a plan view of the scanning pattern utilized in the system of FIG. 3 is shown. A single photomultiplier 20 is employed. As the transparency 18 is moved under the photomultiplier 20, a scan pattern over the surface of the transparency 18 is created along the line indicated by the arrow 44. The relative movement of the photomultiplier 20 is along a first row 46 and then back along a second row 48. That same back and forth scanning continues until finally it traverses the last row 50. The analog to digital conversion which occurs in the quantizer 24 creates intensity data associated with each individual segment (pixel) 47 of each individual row of the transparency 18. For example, in the example of FIG. 2, there are 12 rows and each row is divided into twelve segments 47. Thus, there would be 144 individual values of digital data to describe the transparency. The brightness of each pixel is still continuously variable, but as part of the quantizing process, it is converted into a discrete numerical value.
An improvement to such systems is shown in FIG. 5. A commercial version of the system of FIG. 5 is sold by the assignee of the present application. The single photomultiplier is replaced by a solid state linear photodiode array 52 (such as that sold by E.G.&G. Reticon of Sunnyvale, California). In the solid state scanner 52, a bar 54 contains a row of silicon photo-diodes 56 in close adjacent relationship. The outputs 58 of the photo-diodes 56 are connected to a controller 60 which can output a complete row of pixel values at the output 62 thereof. If the bar 54 is moved over the transparency 18 (or the transparency 18 moved under the bar 54) in the direction of arrows 74, one row at a time can be quantized. This particular environment is another one in which the invention of this application is intended to be used.
Turning to FIG. 6, a prior art control system for use with optical scanners of the type hereinbefore described is shown in simplified form. As shown therein, light 64 striking the surface of a photograph 66 would strike the photomultiplier 20. The electrical output thereof is fed to an amplifier 68. The output of the amplifier is connected to the quantizer 24. The output of the quantizer 24 is connected to the computer 28. An output 70 from the computer is connected back to the amplifier 68 and can be used to set the gain of the amplifier 68 to one of several discrete levels. In such systems, the gain of the amplifier 68 is set to one of its values according to a preselected algorithm in the computer 28 as a function of the value coming from the quantizer 24. Having chosen the amplification value, the quantized value is output at 72 for further use in any manner desired. As can be realized, such a system is rudimentary at best and does not produce an optimized signal value for later use in enhancement. Moreover, when used with a linear photodiode array -type scanner as described with reference to FIG. 5, the problem is compounded.
Wherefore, it is the object of the present invention to provide a control system for optimizing the digital value being generated by a light scanning system as a function of the potential variables being employed.